Examples of very small, small and large tumor burden in patients with persistent disease (PD). On the left side, a 43-year-old female patient with a 40-mm PTC at low-risk after initial surgery (T2NxMx) and very small-volume PD (a-c): post-therapeutic 131I WBS showed a solitary bony focus on the right hip (a, arrow). Fused transaxial image of 131I SPECT/CT (b, arrow) confirmed the bony uptake and hybrid CT (c, arrow) did not display any bone abnormality. On the middle part, a 74-year-old female patient with a 40-mm PTC at low-risk after initial surgery (T2N0Mx) and small-volume PD (d-f): post-therapeutic 131I WBS showed pulmonary metastases (d, red and black arrows). Fused transaxial image (e, red arrow) and hybrid CT scan (f, red arrow) depicted RAI-avid lung micronodules (e-f: 6 mm). On the right side, an 88-year-old female patient with a 40-mm PTC (tall cell variant) at high-risk after initial surgery (T2N1bM1) and large-volume PD (g-i): no abnormal RAI uptake on post-therapeutic 131I WBS with SPECT/CT whereas 18FDG PET/CT showed pulmonary and mediastinal metastases (g, Maximum intensity image, arrows). Fused transaxial image (h, arrow) and hybrid CT scan (i, arrow) showed high 18FDG uptake (SUVmax = 30) by an 18-mm lung nodule.